Motor driven actuator mechanisms for use in controlling the position of valves, dampers, etc. typically include a motor that drives an output coupling in one direction through a gear train to position the valve, damper, etc. in a desired position. Spring type or fail-safe actuator mechanisms also typically include a clock spring coupled to the gear train that is wound during energization of the motor. In this way, energy for rotating the shaft in the other direction when the motor is de-energized is stored in the spring. Upon loss of power to the motor, the clock spring unwinds, driving the gear train to position the valve, damper, etc. in a desired or fail-safe position.
Such mechanisms for rotary actuators are described in U.S. Pat. No. 5,310,021, entitled Motor-Driven, Spring-Returned Rotary Actuator and U.S. Pat. No. 4,595,081 entitled Reversible Rotary Actuator With Spring Return, both of which are assigned to the assignee of the instant application, the teachings and disclosures of which are incorporated in their entireties herein by reference thereto. Another type of spring return system for a linear actuator having particular applicability to drive a valve is described in U.S. Pat. No. 5,529,282, entitled Valve Actuating Device of a Heating and/or Cooling System.
In the rotary-type actuators the motor rotates the output shaft and winds the spring by way of a gear train which substantially reduces the speed and substantially amplifies the torque of the motor. When the spring unwinds to rotate the output shaft, the spring acts reversely through the gear train and backdrives the motor shaft. An actuator of this type is frequently used to drive a utilization device such as a damper in the duct of a heating, ventilating and cooling system. When the motor is de-energized, the spring drives the output shaft in a direction moving the damper to a closed position against a fixed stop. The effectiveness of the seal of the damper against this fixed stop is somewhat a function of the amount of spring force remaining in the clock spring when the damper encounters the stop. If this position is reached when the spring has released all of its stored energy, the quality of the seal against the stop is determined solely on the quiescent mechanical contact between these two surfaces, taking into account the mechanical connection to the motor through the gear train.
While such contact between the damper and the fixed stop may be adequate to stop flow through the damper for many installations, certain installations may require that the seal between the damper and the stop be positively held. That is, there are some installations that require that the damper be able to remain positively closed with increased pressure. Such positive closing force against the fixed stop is particularly desirable in higher pressure installations and in valve operations. Indeed, nearly all installations could benefit from such a positive closing force imparted by the spring to ensure the integrity of the closed position.
To provide such a positive closing force on the damper, valve, etc. driven by the spring return actuator, the output coupling of the actuator is often rotated a few degrees before being connected to the drive shaft of the driven device (e.g., damper, valve, etc.). Such rotation of the output coupling winds the spring to establish a preload. Once a spring preload is established, the output coupling of the actuator is connected to the drive shaft of the driven device that is positioned in its closed or fail-safe position (referred to herein as the zero position). Once connected, the spring imparts the positive preload force on the driven device at its zero position.
Unfortunately, since the output coupling of the actuator is coupled through a torque multiplying gear train, rotation of this output coupling by hand is somewhat difficult. Further, since the return spring also acts through the torque multiplying gear train, holding the output coupling at the preload position while trying to connect this output coupling to the drive shaft of the driven device is also quite difficult.
In the linear-type actuators used to control valve opening and closing, such as that described in U.S. Pat. No. 5,529,282, an output rack is driven by an output pinion gear coupled directly to a motor shaft. When the motor is energized, it rotates the pinion gear in a direction to linearly translate the output rack away from the valve stem. This linear translation also extends two rack bias springs. With the rack withdrawn, a valve stem biasing spring within the valve is then able to expand to open the valve. When the motor is de-energized, the two linear rack biasing springs contract to force the rack against the valve stem. The force of the two rack bias springs is sufficient to overcome the valve stem bias spring, thereby closing the valve.
Unfortunately, once the valve actuator is positioned on the valve body, there is no way to add or otherwise vary a preload on the valve stem. Further, since the rack bias springs are linear springs, a preload force can only be applied by linearly extending the springs, i.e. by linearly translating the rack by repositioning the actuator relative to the valve body. However, this directly reduces the amount of linear travel of the rack that can occur during operation of the valve. Depending on the preload force required, this may result in failure of the valve to open fully because the rack cannot be linearly translated beyond its stop.
There exists, therefore, a need in the art for a linear actuator that includes that ability to apply and adjust a preload force without reducing the linear travel of the output rack, and that provides a manual lockout to enable installation of the linear actuator without difficulty.